Construction of new variants of dextransucrase dsr-s by genetic engineering

ABSTRACT

The present invention relates to a recombinant process for the production of truncated or mutated dextransucrases while conserving the enzymatic activity or their specificity in the synthesis of the α-1,6 bonds. The present invention relates to nucleic acid sequences of truncated or mutated dextransucrases, vectors containing the nucleic acid sequences and host cells transformed by sequences encoding truncated or mutated dextransucrases. In another aspect, the invention concerns a method for producing, in a recombinant manner, truncated or mutated dextransucrases which conserve their enzymatic activity or which conserve their specificity in the synthesis of α-1,6 bonds and can produce, from saccharose, dextrans with high molar mass and modified rheological properties compared with the properties of dextran obtained with the native enzyme and isomalto-oligosaccharides with a controlled molar mass and dextrans. The dextrans and isomalto-oligosaccharides of the invention can be used namely as texturing agents or as prebiotics.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 14/798,518, filed Jul. 14, 2015, which is a divisional of U.S. patent application Ser. No. 13/564,776, filed Aug. 2, 2012 (now U.S. Pat. No. 9,109,207, issued Aug. 18, 2015), which is a divisional of U.S. patent application Ser. No. 12/930,748, filed Jan. 14, 2011 (now U.S. Pat. No. 8,263,380, issued Sep. 11, 2012), which is a divisional of U.S. application Ser. No. 12/223,744, filed Nov. 11, 2008 (now U.S. Pat. No. 7,897,373, issued Mar. 1, 2011), which is a U.S. National Stage Application of International Application No. PCT/IB07/00951 (filed Feb. 8, 2007), claiming priority from FR 06/01117 (filed Feb. 8, 2006), said patent applications hereby incorporated by reference.

SUBMISSION OF SEQUENCE LISTING

The contents of the following submission of the ASCII text file of the Sequence listing (file name: B6624AST25, date recorded: Jun. 14, 2016, size: 152 KB) is submitted via EFS-Web and is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates to a recombinant process for the production of truncated and/or mutated dextransucrases while conserving their enzymatic activity and/or conserving their specificity for synthesizing α-1,6 bonds. More precisely, the present invention relates to nucleic acid sequences of truncated or mutated dextransucrases, vectors containing said nucleic acid sequences and host cells transformed by sequences encoding truncated or mutated dextransucrases. In a further aspect, the invention concerns a method for producing, in a recombinant manner, truncated and/or mutated dextransucrases which conserve their enzymatic activity and/or conserve their specificity for synthesizing α-1,6 bonds in the final product and methods for producing dextrans or isomalto-oligosaccharides, in a single step, with a controlled molar mass and dextrans with modified rheological properties, especially compared with the properties of dextrans obtained with the native enzyme.

FIELD OF THE INVENTION

Dextrans are α-D-glucans with various structures, comprising contiguous glycosyl units more than 50% of which have α-1,6 bonds in the principal chain and α-1,2, α-1,3 and/or α-1,4 branches [1]. The enzymes which produce such dextrans from sucrose are termed dextransucrases and belong to glycoside hydrolase family 70 [2]. During the reaction, fructose derived from the sucrose is released and may be upgraded elsewhere. Dextransucrases are produced by lactic bacteria from genera Leuconostoc, Streptococcus and Lactobacillus [1].

Dextransucrase (DSR-S) from Leuconostoc mesenteroides NRRL B-512F contains 1,527 amino acids [3]. This enzyme catalyzes the synthesis of glucose homopolymers with more than 95% α-1,6 bonds. The production of dextran may be redirected towards that of oligosaccharides or glucosylated conjugates by adding a suitable acceptor to the reaction mixture [4].

The number of industrial applications for dextrans and dextran derivatives is increasing, in particular for dextrans with a specific size. Dextrans with a size in the range 70,000 to 100,000 Da are, for example, used as a plasma substitute [5, 31]. Further, dextran of 40,000 Da is used to improve blood flow, most probably by reducing the viscosity of the blood and inhibiting erythrocytary aggregation [6,8]. After sulphation, smaller dextrans of about 10,000 daltons, for example, are used as transporters for iron [7] or anticoagulants [8]. Those compounds may have antiviral properties [9, 10].

Further, cross-linked dextran derivatives have long been used in the field of molecular separation; chromatography supports under the trade name Sephadex® have been sold since 1961 [6].

Moreover, the European Union has recently approved the use of dextran as a food ingredient in bakery products when these contain more than 95% of α-1,6 bonds and have a molar mass of more than 2×10⁶ Da [15].

Dextransucrase may also produce isomalto-oligosaccharides (IMO) via an acceptor reaction. Acceptor reactions carried out by glucansucrases consist of a transfer of glucosyl residues from sucrose to other molecules added to the reaction medium. It is of increasing commercial interest, particularly in Japan, where the demand for isomalto-oligosaccharides represents about fifteen thousand tons per year [11]. Such small IMOs (DP 2 to 6) are used in bakery items, for drinks, in sake, in seasonings, in confectionery and as anticariogenic sweeteners. It has also been shown that said IMOs have prebiotic properties which are useful with respect to the intestinal and/or vaginal flora [12, 13]. These properties appear to vary with the size of the IMOs and are favored by high degrees of polymerization [14].

The only commercial and usual source of dextrans consists of cultivating L. mesenteroides NRRL B-512F with sucrose, leading to the formation of high molar mass polymers of about 10⁸ Da. The direct synthesis of smaller dextrans of 10000 to 100000 Da is currently impossible. Dextrans are currently produced conventionally by acid hydrolysis of high molar mass native polymers followed by fractionation using organic solvents. This second step is, however, renowned for its low yields [19].

From a commercial viewpoint, IMOs of DP 2 to 6 are not produced by an acceptor reaction with dextransucrase DSR-S and glucose due to the low reaction yields, but from starch hydrolysates and a mixture of α-amylases and glucosidases [11].

Monchois et al [16] describe carboxy-terminal deletions from the dextransucrase of Leuconostoc mesenteroides NRRL B-512F and conclude that the role of the C-terminal domain is to facilitate transfer of dextran and oligosaccharides beyond the active site.

U.S. Pat. No. 5,229,277 describes a process for producing dextran polymers having a homogeneous low molar mass using Leuconostoc mesenteroides and a mutant microorganism of Lipomyces starkeyi ATCC 74054, which is a yeast having dextranase activity, a specific enzyme for the hydrolysis of α-1,6 bonds of dextran. That method necessitates particular culture conditions and a precisely regulated duration and temperature so that the dextranase activity reduces the molar mass of the dextrans. Dextran polymers produced by that method have a molar mass in the range of 40,000 and 150,000 Da.

The foregoing shows that there is a need for the production of dextrans with a molar mass of about 10,000 to 100,000 Da using a faster method with a better yield, which in particular requires neither acid hydrolysis nor fractionation.

The present invention concerns dextransucrases produced in a recombinant manner, which are truncated and/or mutated, while conserving their enzymatic activity and/or conserving their specificity for synthesizing α-1,6 bonds, or truncated variants of dextransucrase which produce dextrans with a controlled molar mass. More precisely, they conserve the binding specificity of native DSR-S and/or conserve their specificity for synthesizing α-1,6 bonds and, starting from sucrose, produce high molar mass dextrans with interesting texturing properties and/or dextrans and IMOs with a controlled molar mass.

The present invention also pertains to providing nucleic acid sequences of truncated and/or mutated dextransucrase, vectors and host cells transformed by said vectors, and amino acid sequences of truncated and/or mutated dextransucrases.

In particular, as will become apparent from the Examples, certain dextransucrases produce polymers with interesting texturing properties, i.e., substantially superior to those of the polymer produced by the native enzyme; others produce dextrans and isomalto-oligosaccharides with a controlled molar mass. Isomaltose is produced by at least one truncated and mutated dextransucrase.

Further aspects of the present invention will become apparent from the following description and Examples or preferred implementations.

SUMMARY OF THE INVENTION

In a first aspect, the invention concerns a nucleotide sequence consisting essentially of or consisting of a nucleotide sequence according to FIG. 1 (SEQ ID NO: 1), a nucleotide sequence according to FIG. 2 (SEQ ID NO: 2), a nucleotide sequence according to FIG. 3 (SEQ ID NO: 3), a nucleotide sequence according to FIG. 4 (SEQ ID NO: 4), a nucleotide sequence according to FIG. 5 (SEQ ID NO: 5), a complementary sequence of one of the sequences with SEQ ID NO: 1, 2, 3, 4 or 5 or a sequence which hybridizes with a sequence with SEQ ID NO: 1, 2, 3, 4 or 5 under stringent hybridization conditions, provided that it conserves dextransucrase enzymatic activity.

In a further aspect, the invention concerns nucleotide sequences of dextransucrase consisting essentially of or consisting of a nucleotide sequence selected from the fragment of SEQ ID NO: 1 from position 373 to position 4269 (SEQ ID NO: 17), the fragment of sequence SEQ ID NO: 2 from position 373 to position 4005 (SEQ ID NO:18), the fragment of sequence SEQ ID NO: 3 from position 373 to position 3408 (SEQ ID NO:19), the fragment of sequence SEQ ID NO: 4 from position 373 to position 3018 (SEQ ID NO:20), and the fragment of sequence SEQ ID NO: 5 from position 373 to position 4269 (SEQ ID NO:21).

It also concerns nucleotide sequences consisting essentially of or consisting of a nucleotide sequence selected from a complementary nucleotide sequence of the fragment of SEQ ID NO: 1 from the nucleotide at position 373 to that at position 4269, a complementary nucleotide sequence of the fragment of SEQ ID NO: 2 from the nucleotide at position 373 to that at position 4005, a complementary nucleotide sequence of the fragment of SEQ ID NO: 3 from the nucleotide at position 373 to that at position 3408, a complementary nucleotide sequence of the fragment of SEQ ID NO: 4 from the nucleotide at position 373 to that at position 3018 and a complementary nucleotide sequence to the fragment of SEQ ID NO: 5 from the nucleotide at position 373 to that at position 4269.

It also concerns nucleotide sequences which hybridize under stringent conditions with a nucleotide sequence selected from the fragment of sequence SEQ ID NO: 1 from position 373 to position 4269, the fragment of sequence SEQ ID NO: 2 from position 373 to position 4005, the fragment of sequence SEQ ID NO: 3 from position 373 to position 3408, the fragment of sequence SEQ ID NO: 4 from position 373 to position 3018 and the fragment of sequence SEQ ID NO: 5 from position 373 to position 4269, provided that it conserves dextransucrase enzymatic activity and said nucleotide sequences that hybridizes thereto has the same number of nucleotides and hybridizes over the full length of the fragment.

In yet another aspect, the present invention concerns nucleotide sequences encoding a protein consisting essentially of or consisting of consecutive amino acid sequences of any one of SEQ ID NOs:6 to 10 or 22 to 26.

In a still further aspect, the present invention concerns vectors, for example plasmids, and host cells transformed by said vectors and containing said sequence of nucleic acids from truncated and/or mutated dextransucrase, in particular the variants of the Examples.

In a still further aspect of the present invention, the present invention concerns a protein encoded by said truncated and/or mutated dextransucrase nucleotide sequence selected from the fragment of SEQ ID NO: 6 from the amino acid at position 125 to the amino acid at position 1423 (SEQ ID NO: 22), the fragment of SEQ ID NO: 7 from the amino acid at position 125 to the amino acid at position 1335 (SEQ ID NO: 23), the fragment of SEQ ID NO: 8 from the amino acid at position 125 to the amino acid at position 1136 (SEQ ID NO: 24), the fragment of SEQ ID NO: 9 from the amino acid at position 125 to the amino acid at position 1006 (SEQ ID NO: 25), and the fragment of SEQ ID NO: 10 from the amino acid at position 125 to the amino acid at position 1423 (SEQ ID NO: 26).

Further, the invention concerns a truncated and/or mutated dextransucrase consisting essentially of or consisting of one of the sequences described here, in particular selected from the fragment of SEQ ID NO: 6 from the amino acid at position 125 to the amino acid at position 1423 (SEQ ID NO:22), the fragment of SEQ ID NO: 7 from the amino acid at position 125 to the amino acid at position 1335 (SEQ ID NO:23), the fragment of SEQ ID NO: 8 from the amino acid at position 125 to the amino acid in position 1136 (SEQ ID NO:24), the fragment of SEQ ID NO: 9 from the amino acid at position 125 to the amino acid at position 1006 (SEQ ID NO:25), and the fragment of SEQ ID NO: 10 from the amino acid at position 125 to the amino acid at position 1423 (SEQ ID NO:26).

In a further aspect, the invention concerns the preparation of a mutated and/or truncated dextransucrase by culture of host cells containing a truncated and/or mutated dextransucrase under conditions allowing the expression of a dextransucrase, and isolating said dextransucrase from the culture medium.

The invention also concerns a method for producing dextrans and/or isomalto-oligosaccharides (IMO) with a controlled molar mass controlled by reacting a mutated and/or truncated dextransucrase of the invention with sucrose and optionally an acceptor, to obtain said dextrans or IMO with a controlled molar mass, including isomaltose.

A method for the direct production of IMOs essentially from sucrose also constitutes an aspect of the invention. The term “essentially” as used here means that it is not necessary for the acceptor to be employed in the reaction.

The high molar mass dextrans of the invention have modified rheological properties compared with those of dextran synthesized by a native enzyme, in particular a non-Newtonian, stringy and/or gelling nature.

Finally, the invention concerns compositions comprising dextrans obtained by using said dextransucrases and the use of said dextransucrases for the production of dextrans and isomalto-oligosaccharides with a controlled molar mass in the range of 342 and 10⁹ Da. More precisely, the invention produces (i) isomaltose (342 Da), (ii) isomalto-oligosaccharides of 342 to 5,000 Da, (iii) dextrans with a controlled size of 1,300 to 52,000 Da, more precisely 5,000 to 22,000 Da, and centered around 10,000 Da, (iv) dextrans with a controlled size of 7,000 to 1.7 x 10⁵ Da, more precisely between 22,000 and 70,000 Da, centered around 40,000 Da.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the amino acid (SEQ ID NO: 6) and nucleotide sequence (SEQ ID NO: 1) of a truncated DSR-S vardel Δ4N dextransucrase with a thioredoxin tag in the 5′ terminal position of the sequence and 6 histidine tags in the 3′ terminal position of the sequence as well as spacer arms between the protein tags and the sequence coding for dextransucrase.

FIG. 2 shows the amino acid (SEQ ID NO: 7) and nucleotide sequence (SEQ ID NO: 2) for a truncated DSR-S vardel Δ3 with a thioredoxin tag in the 5′ terminal position of the sequence and 6 histidine tags in the 3′ terminal position of the sequence and spacer arms between the protein tags and the sequence coding for dextransucrase.

FIG. 3 shows the amino acid (SEQ ID NO: 8) and nucleotide sequence (SEQ ID NO: 3) for a truncated DSR-S vardel Core with a thioredoxin tag in the 5′ terminal position of the sequence and 6 histidine tags in the 3′ terminal position of the sequence and spacer arms between the protein tags and the sequence coding for dextransucrase.

FIG. 4 shows the amino acid (SEQ ID NO: 9) and nucleotide sequence (SEQ ID NO: 4) for a truncated DSR-S Core ΔA with a thioredoxin tag in the 5′ terminal position of the sequence and 6 histidine tags in the 3′ terminal position of the sequence and spacer arms between the protein tags and the sequence coding for dextransucrase.

FIG. 5 shows the amino acid (SEQ ID NO: 10) and nucleotides sequence (SEQ ID NO: 5) for a mutant DSR-S vardel Δ4N SEV663YDA with a thioredoxin tag in the 5′ terminal position of the sequence and 6 histidine tags in the 3′ terminal position of the sequence and spacer arms between the protein tags and the sequence coding for dextransucrase.

FIG. 6 is a diagrammatic representation of the truncated variants of DSR-S and their relative activity. The four different domains (i) to (iv) of DSR-S correspond to: (i) signal peptide, (ii) variable region; (iii) catalytic domain and (iv) C-terminal domain as well as the repeat units A, C and N (in the shaded boxes) located in accordance with Monchois et al, 1998 [16].

FIGS. 7A and 7B respectively show anti-thioredoxin (FIG. 7A) and anti-6×His (FIG. 7B) Western blots carried out on a DSR-S vardel Δ4N produced by E. coli TOP10 at 23° C.

FIG. 8 shows an electrophoresis gel after staining the proteins with colloidal blue on DSR-S vardel Δ4N extracts during affinity purification on nickel resin (Probond, Invitrogen). Track 1 corresponds to the supernatant from sonication of E. coli TOP10 at the end of culture; track 2 corresponds to the effluent obtained after binding the tagged 6×His proteins on the resin, track 3 corresponds to the elution fraction and track 4 corresponds to the elution fraction after eliminating aggregates.

FIG. 9 shows the elution profiles obtained by HPSEC of dextrans produced by the preparation of a) native DSR-S from L. mesenteroides NRRL B-512F, b) entire recombinant DSR-S, c) DSR-S vardel Δ4N before purification and d) purified DSR-S vardel Δ4N. Peak 1 corresponds to the high molar mass polymer (HMW), peak 2 to fructose, glucose and oligosaccharides with a DP of less than 7, not separated by the system. Between those two peaks, perturbations of the base line reflect the presence of dextrans with an intermediate size (between 10³ to 10⁷ Da) in a very low concentration.

FIG. 10 shows spectra obtained by proton NMR on dextrans synthesized by: native DSR-S from L. mesenteroides NRRL B-512F (Spectrum A) the entire recombinant DSR-S (Spectrum B), DSR-S vardel Δ4N before purification (Spectrum C), and DSR-S vardel Δ4N after purification (Spectrum D). Spectrum E is a carbon-13 spectrum of the dextran synthesized by purified DSR-S vardel Δ4N.

FIG. 11 corresponds to the HPAEC-PAD chromatogram of the digestion products using endodextranase (dase) of the four dextrans synthesized by native DSR-S, entire recombinant DSR-S and DSR-S vardel Δ4N, before and after purification.

FIGS. 12A and 12B show the rheological behavior of four dextrans synthesized by native DSR-S (1) before and (2) after shearing, entire recombinant DSR-S (3) before and (4) after application of a second series of shear stresses, DSR-S vardel Δ4N (6) before and (7) after application of a second series of shear stresses, purified DSR-S vardel Δ4N (5) where FIG. 12A represents the measurement of the viscosity flow, and FIG. 12B illustrates dynamic mode viscosity measurements (oscillations between 0 and 10 Pa), before determining the conservation G′ and energy dissipation G″ moduli for the dextrans synthesized by the non-purified DSR-S vardel Δ4N preparations (° and ; solution type behavior, G′<G″; at 5% deformation) and purified preparation (□ and ▪; gel type behavior G′>G″; 0.4% deformation).

FIG. 13 shows a HPAEC-PAD chromatogram of products synthesized by mutant DSR-S vardel Δ4N SEV663YDA with 100 g/l of sucrose alone (A) or by acceptor reaction with 100 g/l of sucrose and 50 g/l of glucose (B). The symbol G signifies glucose, F: fructose, I₂: isomaltose, I3: isomaltotriose, N/M: nigerose or maltose (not separated by the HPAEC-pad system) and the symbol “?” corresponds to products with an unknown structure.

FIG. 14 shows the HPSEC chromatogram of dextrans synthesized by DSR-S vardel Δ3 at 20° C. and 10° C. The arrows correspond to the retention times of commercial dextrans of 2×10⁶ Da, 70,000 and 10,000 Da which served as references.

FIG. 15 shows the HPSEC chromatogram of dextrans synthesized at 20° C. with 100 g/l of sucrose and with 1 U/ml of (1) DSR-S vardel Δ4N, (2) DSR-S vardel Δ3, (3) DSR-S vardel Core and (4) DSR-S Core ΔA and the elution profile (5) of a commercial dextran of 10,000 Da (Sigma).

FIG. 16 shows the HPAEC-PAD profile of dextrans synthesized at 20° C. with 100 g/l of sucrose and with 1 U/ml of DSR-S vardel Δ4N (1), DSR-S vardel Δ3 (2), DSR-S vardel Core (3) and DSR-S vardel Core ΔA (4).

FIGS. 17A and 17B respectively show the HPAEC-PAD profile (FIG. 17A) and distribution (FIG. 17B) of IMOs produced by an acceptor reaction at 20° C. with the variants DSR-S vardel Δ4N (1), DSR-S vardel Δ3 (2), DSR-S vardel Core (3) and DSR-S vardel Core ΔA (4). G: glucose; F: fructose; L: leucrose; T: trehalulose; I2 to I20: isomalto-oligosaccharides with DP 2 to DP 20. The insert of FIG. 17B corresponds to an enlargement of the IMOs from DP of 15 to 27.

DETAILED DESCRIPTION OF THE INVENTION

The term “enzyme having dextransucrase enzymatic activity” as used here means an enzyme which catalyzes the conversion of sucrose into oligosides and polyosides comprising more than 50% glucosyl units bound by α-1,6 bonds with a size in the range 342 and 10⁹ Da, and more particularly dextrans and isomalto-oligosaccharides comprising more than 95% α-1,6 bonds. This conversion may take place in the presence of absence of external acceptors such as maltose, glucose, isomaltose or fructose or isomalto-oligosaccharides. Maltose, isomaltose and glucose are the preferred acceptors in the present invention. The enzymatic activity of the dextransucrases of the present invention may be measured as described in the Examples.

The terms “nucleotides”, “polynucleotides” “nucleic acids” and “oligonucleotides” as used here are interchangeable and include, without being limited thereto, RNA, DNA, DNA/RNA sequences comprising more than one nucleotide in a single chain or in the form of a double chain. The polynucleotide sequences of the present invention may be prepared by any known method including, without being limited thereto, any recombinant synthesis method and any ex vivo generation method, as well as combinations of those methods.

The term “truncated” as used here means that at least one of the N- or C-terminal ends of the amino acid or nucleic acid sequence has been shortened. That shortening may be carried out using restriction enzymes, proteolytic enzymes or synthetically, including by specific amplification of nucleotide sequences, in particular by PCR.

The term “purified dextransucrase” as used here means a dextransucrase which has only one active form of dextransucrase in the preparations, which has a degree of protein purity of at least 70% or 85% or 95%.

The term “interesting original texturizing property” as used here means the rheological properties of the dextrans of the invention which, compared with dextrans synthesized by native enzyme under the same conditions, for example, exhibit non-Newtonian behavior, especially a gel or stringy type behavior. A “gel type polymer” is characterized here by dynamic mode rheological measurements, detecting the energy conservation (G′) and energy dissipation (G″) moduli. For a gel, G′ is higher than G″ over the entire frequency range studied, as will become apparent in Example 5. The stringy character can be identified with the naked eye. The stringy dextrans of the invention change from solution type behavior to gel type behavior after application of a second series of shear stresses, as will also be seen in Example 5.

The following abbreviations used here have the following meanings: DSR-S for dextransucrase from L. mesenteroides NRRL B-512F; DP for degree of polymerization; HMW for “high molar mass”, IMW for “intermediate molar mass”, IMW polymers being highly polydispersed polymers with sizes in the range 1,000 to 10⁷ Da, where separation by HPSEC is difficult because of their low concentration. LMW polymers (low molar mass) are, according to the invention, a population which is much higher and easily detected between 750 and 70,000 Da, centered around 10,000 Da or in the range 2,000 to 1.7×10⁵ Da and centered around 40,000 Da.

The term “10,000 Da dextran” as used here means a population of dextran with a size in the range 1,300 to 52,000 Da, more precisely between 5,000 and 22,000 Da, and centered at the height of the peak at about 10,000 Da. During characterization, the base of the elution peak obtained by gel permeation was in the range 1,300 to 52,000 Da, the range of molar mass estimated at the elution peak half height was in the range from 5,000 to 22,000 Da and the peak was centered at the height of the peak at about a mass of 10,000 Da. When the molar mass was expressed at the peak half height, at least 50% of the dextran population fell within the indicated range.

The term “40,000 Da dextran” as used here means a population of dextran with a size in the range 7000 to 1.7×10⁵ Da, more precisely between 22,000 and 70,000 Da, and centered at the height of the peak at about 40,000 Da. During characterization, the base of the elution peak obtained by gel permeation was in the range 7,000 to 1.7×10⁵ Da, the range of molar mass estimated at the elution peak half height was in the range 22,000 to 70,000 Da and the peak was centered at a mass of about 40,000 Da. When the molar mass was expressed at the peak half height, at least 50% of the dextran population fell within the indicated range.

IMO means isomalto-oligosaccharides.

The term “consisting essentially of” when used in connection with nucleic acids or amino acids as used here means that other minor ingredients or molecules may be present with the amino acid or nucleic acid sequences. The nucleic acid sequence has the exact same length as indicated in the sequence identification number, but may have 3 to 12 extra nucleotides at the N- and C-terminals. Like wise, the amino acid sequence has the exact same length as indicated in the sequence identification number but from 1 to 4 extra amino acids may be added at the N- or C-terminals. These extra amino acids have no effect on the enzyme activity.

More specifically, the present invention concerns nucleic acids which encode a truncated dextransucrase or a mutated dextransucrase, a sequence complementary to all or part of those sequences or a sequence which hybridizes under stringent conditions with one of the above sequences provided that dextransucrase enzymatic activity is maintained. It should be appreciated that the nucleotide sequences that hybridizes thereto has the same number of nucleotides and hybridizes over the full length of the fragment.

The term “stringent hybridization conditions” as used here means conditions as described by Sambrook et al, Molecular Cloning Manual, 3^(rd) edition (2001), i.e., as an example, the following conditions: hybridization buffers: 2×SSC, 10× Denhardts solution (Ficoll 400 & PEG & BSA, ratio 1:1:1), 0.1% SDS, 5 mM EDTA, 50 mM Na₂HPO₄, 250 μg/ml herring sperm DNA, 50 μg/ml of t-RNA or 0.25 M of sodium phosphate buffer with a pH of 7.2, 1 mM EDTA, 7% SDS;

Hybridization temperature: 60° C.;

Washing buffer: 2×SSC, 0.1% SDS;

Washing temperature: 60° C.

The nucleic acid molecules which hybridize under stringent conditions with the nucleic acids of the present invention may in principle encode dextransucrases from any microorganism such as bacteria, gram positive bacteria and, in one aspect of the invention, bacteria from the genera Leuconostoc, Streptococcus or Lactobacillus.

The present invention concerns nucleic acids which encode dextransucrase proteins having at least 70% or 80% or 90% sequence identity with those of sequences SEQ ID NOs: 1 to SEQ ID NO: 5 and SEQ ID NOs 17 to 21:, provided that the protein encoded by said sequences has dextransucrase enzymatic activity.

In another aspect, the present invention concerns nucleotide sequences encoding a protein consisting essentially of or consisting of consecutive amino acid sequences of any one of SEQ ID NOs: 6 to 10 or 22 to 26.

In a further aspect of the invention, the sequences complementary to the sequences of the invention or sequences which hybridize with said sequences under stringent conditions, provided that dextransucrase enzymatic activity is maintained, are also included in the present invention.

Derivations from the basic nucleotide sequences SEQ ID NO: 1, SEQ ID NO: 2, SEQ ID NO: 3, SEQ ID NO: 4 or SEQ ID NO: 5), where the sequences are selected from the fragment of sequence SEQ ID NO: 1 from position 373 to position 4269, the fragment of sequence SEQ ID NO: 2 from position 373 to position 4005, the fragment of sequence SEQ ID NO: 3 from position 373 to position 3408, the fragment of sequence SEQ ID NO: 4 from position 373 to position 3018 and the fragment of sequence SEQ ID NO: 5 of a nucleotide in position 373 to position 4269, the sequences complementary to said sequences or sequences which hybridize with said sequences under stringent conditions provided that the dextransucrase enzymatic activity is maintained, may be produced by deletion, substitution, insertion or recombination, for example; the methods for carrying out said steps and transformations being well known in the art and described, for example, by Sambrook et al, supra.

It should be understood here that if any deletions, substitutions, insertions or recombinations of any of the sequences cited above take place, the proteins encoded by the sequences must maintain their dextransucrase enzymatic activity. Thus, 1 to 132, preferably 2 to 60 nucleotides, more preferably 15 to 36 nucleotides and still more preferably 12 to 27 nucleotides may be modified, for example, by deletion, substitution, insertion or recombination. According to the invention, 90%, preferably 95% of the nucleotides remain unchanged.

The dextransucrase enzymatic activity can be measured, as described in the method section and in the Examples of the present application.

The oligonucleotides which may be used as a probe or primer are, for example, SEQ ID NO: 1 to SEQ ID NO: 5 or nucleotide sequences selected from the fragment of sequence SEQ ID NO: 1 from position 373 to position 4269, the fragment of sequence SEQ ID NO: 2 from position 373 to position 4005, the fragment of sequence SEQ ID NO: 3 from position 373 to position 3408, the fragment of sequence SEQ ID NO: 4 from position 373 to position 3018 and the fragment of sequence SEQ ID NO: 5 from a nucleotide in position 373 to position 4269.

The length of the probes and primers can vary depending on their applications. In general, they must have at least 25 nucleotides and may comprise all of the dextransucrase sequences described, such as 3,896 nucleotides. The length can also vary to be in the range of 25 to 150 nucleotides, 25 and 800 nucleotides or 25 and 3000 nucleotides, for example.

The primers generally comprise 18 to 25 nucleotides in length, but may also be longer, depending on the envisaged application. Examples of primers which can be used in the present invention are:

(SEQ ID NO: 11) GGC TTC TCT GGT GTG ATT (SEQ ID NO: 12) GAT CTG TCA GAA ACT GGC (SEQ ID NO: 13) ACA CAA CAA GTT AGC GGC (SEQ ID NO: 14) CCA GAT ACT AAC TTG AGT (SEQ ID NO: 15) TTC ATT GAT GCA GAC GGG (SEQ ID NO: 16) CAC GAC TAC GAC GCG CAA

It should be noted that the primers in the 5′ and 3′ terminal positions of the nucleotides encode the dextransucrase (SEQ ID NOs: 11 to 15) and the 5′ and 3′ side of the mutant sequence (SEQ ID NO: 16). However, a skilled person can use each of these sequences to produce primers or probes using consecutive nucleotides. Furthermore, these nucleotide sequences which are used as a probe may be tagged with radioactivity, enzymatic tagging, fluorescent tagging, in particular.

In order to genetically engineer the prokaryotic or eukaryotic cell, the nucleic acids of the present application or a portion of the nucleic acids of the present application may be introduced into plasmids that allow mutagenesis or modification of sequences by recombination of nucleotide sequences. Standard methods using these techniques are known to the skilled person and have been described by Sambrook et al, supra, in particular. The DNA fragments can also be connected to each other by adapters or links and suitable restriction enzymes can be used to remove certain DNA sequences. Methods such as mutagenesis, restriction after the restoration of primers or ligatures can be used to obtain the desired sequence with the appropriate insertions, deletions or necessary or desirable substitutions.

Furthermore, well defined tags coding for nucleic acids may be attached to the N- or C-terminal ends of the nucleic acid sequences of the present invention. They may be peptides such as poly-His, c-myc epitope or HA-tag or small proteins such as bacterial GST, MBP (maltose binding protein), thioredoxin, β-galactosidase, VSV-glycoprotein and the like.

Particular nucleic acids coding for other protein tags are His-tag, T7tag, S-tag, a “flag” peptide, trpE, avidin/streptavidin, staphylococcal A or G protein, dihydrofolate reductase, cellulose binding domains, polycysteine, polyphenylalanine and the like, which may also be used in the present invention.

According to one aspect of the present invention, a nucleic acid coding for a thioredoxin is fused to the N-terminal nucleic acid sequence. A nucleic acid coding for a 6×His tag is fused to the 3′ end of the nucleic acid sequences.

The nucleic acids of the present invention may be linked to a transcription unit comprising (1) gene expression regulation elements such as promoters and amplifiers and (2) a coding or structural sequence which is transcribed into a mRNA and translated into the corresponding protein, and (3) appropriate initiation and termination signals.

A number of suitable expression control sequences are known in the art. General methods for expressing the recombinant protein are also known and exemplified in the document by R Kaufman, Methods in Enzymology 185, 537-566 (1990) [17].

The promoter regions which can be used in the vectors of the present invention include lacL, lacZ, T3, T7, gpt, lambda PR, tre and ara.

The present invention also concerns vectors, in particular plasmids, cosmids, viruses, bacteriophages and other vectors which are known in the genetic engineering field and which comprise the nucleic acid sequences of the present application in one aspect of the present invention, said vectors being plasmids and selected from DSR-S vardel Δ4N, DSR-S vardel Δ3, DSR-S vardel Core, DSR-S Core ΔA and DSR-S vardel Δ4N SEV663YDA.

The nucleic acids of the present invention may be expressed in prokaryotic or eukaryotic cells. Non-limiting examples of such cells which may be cited are VERO cells, HELA cells such as ATCC No CCL3, CHO cell lines such as ATCC CCL61, COS cells such as COS-7 and ATCC No CR cells: 1650, W138, BHK, HepG2, 3T3 such as ATCC No CRL6361, A549, PC12, K562, 293 cells, Sf9 cells such as ATCC No CRL 1711, Cv1 cells such as ATCC No CCL70 and JRKAT cells such as ATCC Tib152.

Non-limiting cells which can be used in the present application include strains of the prokaryotic host cells such as Escherichia coli, Bacillus subtilis, Salmonella typhimurium or strains of the genus Pseudomonas, Streptomyces and Staphylococcus or strains of eukaryotic host cells such as the parasites Apicomplexan (Plasmodia, Toxoplasma, Cryptosporidia), Leishmania or Trypanosoma.

Other appropriate cells may be used in the present invention and in particular include yeast cells such as Saccharomyces, for example Saccharomyces cerevisiae or pombe, Pichia pastoris and eukaryotic cells (plant cells, CHO cells and the like).

In a further aspect, the cells used for expressing nucleic acids of the present invention are Escherichia coli and strains selected, for example, from JM109, BL21(DE3)pLysS, TOP10 or Pir1. The INVsc strain of Saccharomyces cerevisiae may also be used.

The present invention concerns host cells transformed with the nucleic acid sequences described above or with a vector as described above and cells derived from transformed cells and containing the vector or the nucleic acid sequences described herein.

Examples of such host cells which may be cited are Escherichia coli, in which the truncated and/or mutated dextransucrase may be produced. The preparation of such host cells is known in the art.

Proteins and biologically active fragments of such proteins as well as mutated proteins which are encoded by the nucleic acid molecules of the present invention and their preparation methods also fall within the scope of the present invention.

Thus, the present invention concerns a method for preparing mutated and/or truncated dextransucrase, comprising the following steps:

-   -   (a) culturing host cells transformed with the nucleic acid         sequences described above or with a vector as described above         under conditions allowing the expression of a dextransucrase;         and     -   (b) isolating said dextransucrase from the culture medium.

More specifically, the nucleic acid sequences may be selected from SEQ ID NO: 1 from position 373 to position 4269, the fragment of sequence SEQ ID NO: 2 from position 373 to position 4005, the fragment of sequence SEQ ID NO: 3 from position 373 to position 3408, the fragment of sequence SEQ ID NO: 4 from precursor 373 to position 3018, and the fragment of sequence SEQ ID NO: 5 from position 373 to position 4269, complementary sequences of said sequences and sequences which hybridize with said sequences under stringent conditions, provided that dextransucrase enzymatic activity is maintained.

After being isolated, the dextransucrases of the present invention may also be purified. In this respect, the usual purification methods may be used such as precipitation, ion exchange chromatography, affinity chromatography, hydrophobic exchange chromatography, gel filtration, reverse phase HPLC, phase demixing and the like. In one aspect of the present invention, the mutated or truncated dextransucrases of the present invention may be purified using a resin charged with nickel, taking into account the existence of the thioredoxin and 6×His tag.

Another aspect of the present invention concerns dextransucrase proteins consisting essentially of or consisting of an amino acid sequence selected from SEQ ID NO: 6 to 10 or an amino acid sequence selected from the fragment of SEQ ID NO: 6 from the amino acid at position 125 to the amino acid at position 1423, the fragment of SEQ ID NO: 7 from the amino acid at position 125 to the amino acid at position 1335, the fragment of SEQ ID NO: 8 from the amino acid at position 125 to the amino acid at position 1136, the fragment of SEQ ID NO: 9 from the amino acid at position 125 to the amino acid at position 1006, and the fragment of SEQ ID NO: 10 from the amino acid at position 125 to the amino acid at position 1423.

A protein encoded by one of nucleotide sequences SEQ ID NO: 1 to SEQ ID NO: 5 or fragments of said sequences, as set forth above, is another embodiment of the present invention.

Homologous amino acid sequences, i.e., wherein the degree of similarity with the sequences defined above is sufficient for the enzymatic activity to be maintained, are also included in the subject matter of the present application. Thus, Blast and Fasta programs may be used to investigate similarity. Since it was demonstrated herein that it was possible to truncate the N- and C-terminal ends of dextransucrases, maintaining enzymatic activity, sequence similarity cannot be considered for just the single complete sequence, but also for the truncated sequences. The present invention thus concerns any sequence containing 80%, 90% or 98% sequence similarity with the complete sequence, but also those which would have 80%, 90% or 98% sequence similarity with one of the truncated sequences, provided that enzymatic activity is maintained.

More specifically, the present invention concerns sequences having a degree of similarity of the order of 90%, 95% or 98% similarity with SEQ ID NO: 6 to 10 or amino acid sequences selected from the fragment of SEQ ID NO: 6 from the amino acid at position 125 to the amino acid at position 1423, SEQ ID NO: 7 from the amino acid at position 125 to the amino acid at position 1335, SEQ ID NO: 8 from the amino acid at position 125 to the amino acid at position 1136, SEQ ID NO: 9 from the amino acid at position 125 to the amino acid at position 1006, and SEQ ID NO: 10 from the amino acid at position 125 to the amino acid at position 1423, provided that these proteins have the enzymatic activity of said dextransucrases. Clearly, the amino acid sequences with a specific identity defined above have a majority of conservative amino acid substitutions.

Conservative amino acid substitutions include amino acid substitutions of the same class. These classes comprise, for example, amino acids having uncharged polar side chains, such as Asn, Gln, Ser, Thr or Tyr; amino acids containing basic side chains, such as His, Lys or Arg; amino acids containing acidic side chains, such as Glu or Asp and amino acids containing non-polar side chains, such as Ala, Gly, Leu, Val, Ile, Phe, Cys or Trp.

Furthermore, concerning the enzymatic activity of dextransucrase with amino acid substitutions, this can be tested as set forth in the Examples, but the activity can also be evaluated by HPLC analyses or using the usual predictions concerning the way amino acid changes affect protein functions.

In a further aspect, since the amino acid sequences are indicated here, the protein may be synthesized using R B Merrifield's method, 1963 [20]. For this reason, the synthesized dextransucrase proteins constitute another aspect of the present invention.

The present invention also concerns mutant dextransucrases designated mutant SEV663YDA of DSR-S vardel Δ4N in which the serine, glutamic acid and valine in positions 663, 664 and 665 have been modified to tyrosine, aspartic acid and alanine respectively.

This mutant may be used to synthesize isomaltose from sucrose, using sucrose as the only substrate in a yield which is equivalent to that obtained when an acceptor, such as glucose is added to the reaction medium.

For this reason, the present invention concerns a method for producing isomaltose directly from sucrose, said method comprising reacting mutant dextransucrase with SEQ ID NO: 10 with sucrose, and producing isomaltose.

The fusion proteins containing a protein tag as described above also form part of the present invention. In this regard, the mutated and/or truncated proteins of the present invention may be fused with at least one protein tag.

The preparation of high molar mass dextrans (about 10⁶-10⁸ Da) and with modified rheological properties compared with dextran synthesized by native DSR-S of L. mesenteroides NRRL B-512F using the truncated dextransucrase of the present invention is another aspect of the invention.

More specifically, microorganisms secreting dextransucrase or cellular extracts of microorganisms producing dextransucrase in an intracellular manner may be cultivated or used in a medium comprising sucrose, resulting in the synthesis of isomaltose (342 Da), (ii) isomalto-oligosaccharides of 342 to 5,000 Da, (iii) dextrans with a controlled size of 1,300 to 5,200 Da centered around 10,000 Da, (iv) dextrans with a controlled size of 7,000 to 1.7×10⁵ Da centered around 40,000 Da, and (v) dextrans with a high molar mass from 2×10⁶ Da to 10⁹ Da. These compounds may be isolated from the culture medium by conventional methods such as ultrafiltration, nanofiltration, alcoholic precipitation, liquid chromatography and the like.

Alternatively, the truncated and/or mutated dextransucrases described in the present invention may be purified and used in a method for producing dextrans with a controlled molar mass.

Thus, the invention concerns a method for producing dextrans and/or isomalto-oligosaccharides with a controlled molar mass, comprising reacting a mutated and/or truncated dextransucrase consisting essentially of or consisting of a sequence selected from nucleotide sequences SEQ ID NO: 6 to SEQ ID NO: 10 defined above with at least sucrose and optionally an acceptor.

The invention also concerns a method for producing isomaltose, the method comprising reacting a mutated and/or truncated dextransucrase with sequence SEQ ID NO: 10 essentially with sucrose. The invention also concerns a method for producing dextrans with interesting textural properties, the method comprising reacting a mutated and/or truncated dextransucrase with the sequence of SEQ ID NO: 6.

The invention also concerns dextrans and isomalto-oligosaccharides having the characteristics defined in the present application which may be obtained by the methods described here. These characteristic properties include the fact that high molar mass dextrans have non-Newtonian behavior and have the character of a gel or a stringy nature, and the property of changing form a solution type behavior to that of a gel after application of a second series of shear stresses.

As will become apparent in the Examples, advantageously, the different rheological properties may be obtained depending on whether the enzyme is purified or non-purified.

The enzymatically produced dextrans of the invention may be used as a support in the pharmaceutical industry, as a plasma substitute, additives in textiles or paints, in cosmetics and in the agroalimentary industry, as well as a texturing agent, for example as a substitute for gum Arabic or a gelling agent. The invention also concerns compositions comprising the dextrans and IMOs of the invention.

One important application of the dextrans and isomalto-oligosaccharides of the present application is their use as prebiotics. These products are not completely metabolized and are selectively fermented in the colon by appropriate bacterial species such as Bifidobacteria and Lactobacilli.

Oligosaccharides have traditionally been used for human or animal foodstuffs, in the pharmaceutical industries and in the cosmetics industry or as a sweetener, stabilizer or filler [21]. During the last fifteen years, a new field of activity has developed for the prebiotic properties of certain non digestible molecules [23]. Oligosaccharides as prebiotics are interesting with respect to their capacity to resist attack by digestive enzymes and to accentuate the growth of “healthy” bacteria, primarily Bifidobacteria and Lactobacilli, in the intestine. This concept has been stimulated by the emergence of commercial prebiotic products which have rapidly gained popularity. Oligomers such as fructo-oligosaccharides, lactulose, galacto-oligosaccharides, xylo-oligosaccharides, oligosaccharides extracted from soya or isomalto-oligosaccharides which are usually obtained by biological processes or by extraction from plants, are also promising. Currently, research in this field has centered on the production of novel oligosaccharide structures termed second generation prebiotics which should have novel physico-chemical properties and more specific biological activities 1181.

In a further aspect, the present invention concerns a composition comprising a dextran obtained from a dextransucrase of the invention, and a pharmaceutically acceptable vehicle or a food quality vehicle.

The acceptable vehicle may, for example, be selected from adjuvants, salts and the like and the adjuvants may be selected from muramyl peptides, alum, montanide and the like. The mutated and/or truncated dextransucrases may be a purified protein, a protein produced in a recombinant manner or a synthetically produced protein.

Regarding the method for producing the dextrans and/or IMOs, preferred acceptors, when used, are glucose, isomaltose, maltose and isomalto-oligosaccharides.

Preferably, the method for producing isomalto-oligosaccharides with a controlled molar mass comprises reacting a mutated and/or truncated dextransucrase consisting of sequences SEQ ID NO: 7, 8, 9 or 10 essentially with sucrose. The degree of polymerization thus varies from 2 to 60 glucosyl units (DP2 to DP60).

The production reaction takes place at temperatures in the range 4° C. to 80° C., preferably 4° C. to 40° C.

Preferably, when the sequence is SEQ ID NO: 7, SEQ ID NO: 8 or SEQ ID NO: 9, the temperature is in the range 4° C. to 15° C., preferably 8° C. to 12° C., and more preferably the temperature is of the order of 10° C. for the production of dextrans with a controlled size. Further, for such sequences, the temperature is preferably in the range from about 8° C. to 25° C., more preferably on the order of 20° C. for IMO synthesis.

Furthermore, preferably when the sequence is SEQ ID NO: 6 or SEQ ID NO: 10, the temperature is in the range 15° C. to 45° C., preferably 17° C. to 30° C., and more preferably on the order of 20° C. to 25° C.

Further, the sucrose concentration is in the range 10 to 600 g/l, preferably 75 to 400 g/l, and more preferably 90 to 280 g/l.

When the sequence is SEQ ID NO: 7, SEQ ID NO: 8 or SEQ ID NO: 9, the concentration of sucrose in the medium is preferably on the order of 250 g/l.

Further, when the sequence is SEQ ID NO: 6 or SEQ ID NO: 10, the concentration of sucrose may be on the order of 100 g/l.

Further, as appropriate, the sucrose/acceptor weight ratio may be on the order of 0.5 to 12, preferably 1 to 4, more preferably about 2.

In the method of the invention, the dextransucrase is in the free form or immobilized on a support. Said immobilization may be effected by adsorption, inclusion or covalent binding, for example.

Finally, to carry out the method, the pH is in the range 3.0 to 10.0, preferably 4.0 to 7.0, more preferably 4.5 to 6.0 and still more preferably about 5.2.

Other aspects of the invention may become apparent from a study of the Examples below.

EXAMPLE 1 Construction of Variants

The pBad/TOPO Thiofusion vector (Invitrogen) was used for cloning and expressing truncated and/or mutated dsrS genes under the control of the L-arabinose promoter. It allows fusion of the gene to the 6×His tag at the C-terminal end, and to a thioredoxin tag at the N-terminal end.

For use as a matrix, genomic DNA from L. mesenteroides NRRL B-512F was extracted using the “Blood and Cell culture DNA maxi” kit (Qiagen). The strain is derived from the NCAUR collection, Peoria, Ill., USA.

One Shot TOP10 cells (Invitrogen) were used for expression of the truncated and/or mutated dsrS genes. The restriction enzymes were purchased from New England Biolabs and used in accordance with the manufacturer's instructions. DNA was purified using “QIAquick” (purification by PCR and gel extraction) and “QIAprep” (plasmid purification) kits from Qiagen.

The variants were constructed by PCR amplification of the DSR-S gene from genomic DNA from L. mesenteroides NRRL B-512F using the “Expand High fidelity” polymerase (Roche) and the following primers (given in the 5→3′ direction):

-   -   1 DSR-S vardel Δ4N was constructed using the pBad and DSR-S         vardel primers: 454-acacaacaagttagcggcaagtacgttgaaaaagac-490 and         PBad Δ4N: 4350-actcaagttagtatctggatccacaatgatagc-4317. It         contained amino acids T152 to 51450 of DSR-S.     -   2 DSR-S vardel Δ3 was constructed using the PBad and DSR-S         vardel primers: 454-acacaacaagttagcggcaagtacgttgaaaaagac-490 and         PBad Δ3: 4086-cccgtctgcatcaatgaattcacc-4062. It contained amino         acids T152 to G1362 of DSR-S.     -   3 DSR-S vardel Core was constructed using the PBad and DSR-S         vardel primers: 454- acacaacaagttagcggcaagtacgttgaaaaagac-490         and PBad Core: 3489-gccagtttctgacagatcattagttaactg-3459. It         contained amino acids T152 to G1162 of DSR-S.     -   4 DSR-S Core ΔA was constructed using the PBad DSR-S cat         primers: 843-ggcttctctggtgtgattgatggtcaa-870 and PB ad Core:         3489-gccagtttctgacagatcattagttaactg-3459. It contains amino         acids G282 to G1162 of DSR-S.     -   5 The mutant DSR-S vardel Δ4N SEV663YDA was constructed by         directed mutagenesis using the “mega primer” technique [33, 21]         and DNA polymerase Pfu (Strategene). A first PCR reaction was         carried out using the DSR-S vardel Δ4N plasmid matrix and the         SEV663YDA primer pair:         1965-agctttgtacgagctcacgactacgacgcgcaaacggtt-2004 and rev:         3447-gtcaccatcctcagtgttcgaaacg-3422, comprising the BstBI         restriction site (underlined). This PCR product was then used as         a reverse mega primer in a second PCR with the forw primer:         1329-caaccacagtggaatgaaactagtc-1354 comprising the SpeI         restriction site. This second PCR product was then digested with         the two restriction enzymes SpeI and BstBI in accordance with         the manufacturer's conditions (New England Biolabs) and cloned         into the pBad DSR-S vardel Δ4N vector previously digested with         the same enzymes. The SEV663YDA primer was designed to introduce         a single restriction site to select positive clones (in this         case, the SacI site).

The primary structure of each of the variants DSR-S vardel Δ4N, DSR-S vardel Δ3, DSR-S vardel Core and DSR-S Core ΔA is diagrammatically shown in FIG. 6.

EXAMPLE 2 Production of Variants in E coli

Cultures were carried out in a baffled Erlenmeyer flask on 2× YT medium buffered to a pH of 6.4 with 100 mN of Tris-HCl, DSR-S being known to be unstable under alkaline pH conditions [3].

Composition of Medium 2× YT:

Bactotryptone 16 g/l

Yeast extract 10 g/l

NaCl 5 g/l

Tris 12.1 g/l

E. coli TOP10 cells carrying pBad DSR-S vardel Δ4N and pBad DSR-S vardel Δ4N SEV663YDA plasmids were cultivated at 23° C. L arabinose induction was carried out when cell growth reached OD_(600 nm) of 0.2 with 0.002% (w/v) of inducer. Culturing was stopped when cell growth reached a plateau (OD_(600 nm) of about 3-3.5) before starting the cell lysis phase.

E. coli TOP10 cells carrying pBad DSR-S vardel 43, pBad DSR-S vardel Core and pBad DSR-S Core ΔA plasmids were brought to 16° C. Induction was carried out when the cell growth reached OD_(600 nm) of 0.2 with 0.005% (w/v) of L arabinose in the case of DSR-S vardel Δ3 and 0.02% (w/v) in the case of DSR-S vardel Core and DSR-S Core 4A. Culturing was halted when the cell growth reached a plateau (OD_(600 nm) of about 2.5) before starting the cell lysis phase.

Following culture, the cells were recovered by centrifuging (8,000×g, 10 minutes, 4° C.), re-suspended and concentrated to an OD_(600 nm) of equivalent to 80 in a sodium acetate buffer 50 mM, pH 5.2, supplemented with 0.05 g/l of 1 mM CaCl₂ and phenylmethanesulfonyl fluoride (PMSF). Cell rupture was carried out by sonication. The preparations were then centrifuged once again (20,000×g, 30 mM, 4° C.) to eliminate cellular debris and recover only the sonication supernatant.

The enzymatic activity of the extracts was measured using the dinitrosalicylic acid (DNS) method of Sumner and Howell, 1935 [22]. An enzymatic unit is defined as the quantity of enzyme which catalyses the formation of one mole of fructose per minute at a given temperature (4° C. to 40° C. depending on the case, more precisely 20° C. or 30° C.) and in a sodium acetate buffer (50 mM), pH 5.2, containing 0.05 g/l of CaCl₂ and 100 g/l of sucrose.

EXAMPLE 3 Purification of DSR-S Vardel Δ4N Variant

Different enzymatic forms of DSR-S vardel Δ4N were produced during the culture of E. coli TOP10: a vastly major entire form and different degraded forms at the C-terminal end (FIG. 7). The origin of these degradations remains unclear. Production in Example 2 reached about 5500 U/l of culture in the sonication supernatants (activity assayed at 30° C.).

To determine the number of active enzymatic forms in the extracts, electrophoresis gels were produced under native or denaturing conditions. After gel re-naturing, it was incubated overnight at 25° C. in a sodium acetate buffer, 50 mM, pH 5.2 supplemented with 100 g/l of sucrose. The active enzymatic forms then synthesized polymer at the region to which they migrated in the gel. A reagent (Schiff's reagent) which specifically colored the polymers synthesized by active dextransucrases, after oxidation of primary alcohol functions of the periodic acid polymer was used and the gels were stained with this reagent. This type of gel is termed a zymogram. In the case of DSR-S vardel Δ4N, or its mutant SEV663YDA, only the two higher molar mass forms were detected as being active (results not shown). However, only the entire form had both the thioredoxin tag and the 6×His tag.

The presence of the 6×His tag only in the entire form of DSR-S vardel Δ4N was exploited to purify the enzyme by affinity chromatography on nickel resin (Probond Ni-NTA, Invitrogen).

Purification was carried out at 4° C. All of the buffers had concentrations of 50 mM sodium acetate, 400 mM of NaCl, different concentrations of imidazole and were adjusted to a pH of 7.5. The resin was equilibrated with 8 volumes of buffer having a concentration of 40 mM of imidazole. Fixing was carried out for 2 hours with 7 volumes of enzymatic extract supplemented with 20 mM of imidazole and adjusted to a pH of 7.5. Next, the resin was washed with 40 volumes of 40 mM imidazole buffer, 8 volumes at 60 mM and 4 volumes at 100 mM. Finally, the proteins were eluted with 7 volumes of buffer having a concentration of 250 mM of imidazole.

The fractions containing the eluted fusion proteins were mixed and dialyzed overnight at 4° C. against a buffer containing a concentration of 50 mM of sodium acetate, pH of 5.2, and 0.05 g/l of CaCl₂. The protein concentration was determined by the microbradford method (Biorad Laboratories) with BSA (bovine serum albumin) as the standard.

The purity of the preparation at the end of the procedure was estimated at about 90% (FIG. 8). The purified DSR-S vardel Δ4N proteins had a very strong tendency to aggregate, causing the formation of white precipitates and limiting the yields obtained at the end of the procedure (Table 1). However, the specific activity of the preparation was estimated at 584 U/mg of protein, which corresponded to the best described specific activity of a recombinant dextransucrase. By way of comparison, the specific activity of native DSR-S (expressed by L. mesenteroides NRRL B-512F) was estimated at about 170 U/mg [24].

TABLE 1 Purification of DSR-S vardel Δ4N by affinity chromatography on nickel resin Protein Specific Purification Volume Activity conc activity Purification Yield stage (ml) (U/ml) (mg/l) (U/mg) factor (%) Sonication 150 149.2 9.46 15.7 1 100 supernatants Elution 150 67.6 0.25 270.5 17 45.3 fraction after dialysis Soluble 150 38.2 0.09 424.4 27 25.4 fraction after eliminating aggregates

EXAMPLE 4 Nucleotide Sequences and Amino Acid Sequences

The constructs were sequenced and the corresponding sequences are shown in FIGS. 1 to 5.

EXAMPLE 5 Synthesis of Dextran by DSR-S vardel Δ4N, Comparison with DSR-S from L. mesenteroides NRRL B-512F

Dextran was synthesized from native DSR-S from L. mesenteroides NRRL B-512F, entire recombinant DSR-S (sonication supernatant) and DSR-S vardel Δ4N (sonication supernatant and purified enzyme).

Synthesis Conditions and Analysis of Products Formed

Entire recombinant DSR-S was constructed on the same principle as the variants described in Example 1, with primers which were suitable for amplification of the entire gene. E. coli TOP10 cells carrying the pBad DSR-S plasmid were cultivated using the protocol described for DSR-S vardel Δ4N (Example 2). The supernatant contained three enzymatic forms, including two with higher active molar mass.

The form with the greatest size contained DSR-S in its entirety; the two other forms were degraded at their N-terminal position (data not shown).

The activity of each enzymatic preparation was determined at 30° C.

Dextran syntheses were carried out at 25° C. starting with a 100 g/l sucrose solution, in a 50 mM sodium acetate buffer containing 0.05 g/l of CaCl₂ and with 1 unit per ml of enzyme. The progressive exhaustion of sucrose was monitored by HPAEC-PAD analyses (see below) and the reaction was stopped after its complete consumption, by heating for 5 mM at 95° C. (complete denaturing of cited dextransucrases).

The products formed were analyzed by HPAEC-PAD (high performance anion exchange chromatography with pulsed amperometric detection) with respect to the mono, di and oligosaccharides, and by HPSEC (high performance size exclusion chromatography) with respect to the polysaccharides.

The HPAEC-PAD system comprised a Dionex “Carbopack PA100” 4×250 mm column A 6 to 300 mM sodium acetate gradient in 28 minutes in a 150 mM sodium hydroxide solution was applied at a flow rate of 1 ml/min Detection was carried out by amperometry using a Dionex ED40 module with a gold electrode and an Ag/AgCl pH reference electrode.

The HPSEC system was constituted by two Shodex OH-Pack SB-805 and SB-802.5 columns in series, using 0.45 M sodium nitrate+1% (v/v) ethylene glycol as the solvent, in an amount of 0.3 ml/min The columns and pre-columns were kept at 70° C. and the samples were filtered on 0.45 μm filters (Sartorius) prior to injection. Detection was of the refractometric type, coupled to a light diffusion detector (Wyatt) to determine the mass of the dextrans.

The concentrations by weight of glucose, fructose and leucrose (sucrose isomer) were determined by HPAEC-PAD analyses. The percentages of glucosyl residues from the sucrose incorporated into the free glucose and leucrose were calculated using the following formula:

% G _(glucose)=[glucose_(tf)]/([sucrose_(t0)]×(180/342))

and

% G _(leucrose)=[leucrose_(tf)]/[sucrose_(t0)]

where [glucose_(tf)] and [leucrose_(tf)] correspond to the final concentrations of glucose and leucrose at the end of the reaction and [sucrose_(t0)] corresponds to that of the initial substrate (g/l).

The percentage of glucosyl residues incorporated into the HMW polymer was determined by HPSEC analyses using the formula:

% G _(dextran)=surface area_(dextran-tf)/(surface area_(sucrose-t0)/(162/342))

in which surface area_(dextran tf) corresponds to the surface area of the dextran peak, determined using the HPSEC chromatogram at the end of the reaction, and surface area_(sucrose-t0) corresponds to that of the peak of the initial substrate. For a given concentration, the surface obtained by refractometry is identical regardless of the sugar.

The proportion of glucosyl units incorporated into the IMW polymers or oligosaccharides for which the concentration could not be directly quantified by HPAEC-PAD or HPSEC was determined using the formula:

% G _(IMW)=100−% G _(glucose-tf)−% G _(leucrose-tf)−% G _(dextran-tf)

The elution profiles of the four dextrans obtained by HPSEC are shown in FIG. 9. Different populations can be distinguished: a first peak eluted at 38 minutes, corresponding to the high molar mass polymer (HMW), and a second peak at 75 minutes corresponding to fructose, glucose, leucrose (5-O-α-D glucosyl fructose) and other oligosaccharides with a degree of polymerization (DP) of less than 7, not separated by the system or in very low concentrations. Between these two principal peaks, as indicated by the base line perturbations, the products of intermediate size (IMW dextrans) were also present. These compounds, with very variable sizes, between 1000 and 10⁷ Da, were highly polydispersed and in very low concentrations, which explains their low intensity on the chromatogram. HPAEC-PAD analyses confirmed their presence, however (results not shown).

The relative quantity of glucosyl units derived from sucrose and incorporated into the different products is listed below in Table 2. The synthesis yield for HMW dextran represents about 60% of the glucosyl units for each of the preparations. The transfer of glucosyl units to water (glucose) or fructose (leucrose) represents less than 8%, while the synthesis of intermediate size dextrans (IMW) accounted for 25% to 32% of the transferred glucosyl units. All of the recombinant forms of DSR-S tended to synthesize more intermediate size dextrans. The HPSEC analyses also showed that the native enzyme appeared to synthesize two different populations of dextran, as opposed to only one for the recombinant enzymes. The molar mass of HMW dextrans was determined by light diffusion and estimated to be over 10⁷ g/mol for all of the samples (exclusion limit of the columns used).

TABLE 2 Percentage of glucosyl units incorporated into the various products derived from the synthesis of dextran at 25° C. and 100 g/l of sucrose, for the four cited DSR-S preparations HMW dextrans IMW HMW Glucose Leucrose dextrans Rel % (g/mol) Native 4.12 5.80 25.60 64.47  1.5 × 10⁸ DSR-S 8.88 × 10⁷ Entire DSR-S 2.32 5.39 29.32 62.96 1.86 × 10⁸ DSR-S 2.43 5.90 31.03 60.64 4.87 × 10⁷ vardel Δ4N Purified 2.33 5.80 32.24 59.62 2.47 × 10⁷ DSR-S vardel Δ4N

Structure of Dextrans Formed

The structure of the dextran produced by DSR-S vardel Δ4N (purified or otherwise) was compared with that of dextrans synthesized from entire recombinant DSR-S and native DSR-S. These structures were determined by nuclear magnetic resonance (¹H NMR) using a Brucker AC 300, at 85° C. and with an acquisition frequency of 300.13 MHz. The acquisition time was 3 s, with 32 to 64 passes. The dextrans were initially separated from the co-produced fructose by precipitating 3 times with 1 volume of absolute ethanol, recovered by centrifuging, washed with distilled water and freeze dried. The samples were dissolved in D₂O to a concentration of 6 mg/ml.

The NMR spectra are shown in FIG. 10. Only α-1,6 bonds were detected. Carbon-13 NMR analysis was also carried out on the dextran synthesized by purified DSR-S vardel Δ4N. The spectrum obtained was identical to those published for the dextran from L. mesenteroides NRRL B-512F and entire DSR-S [3].

These polymers were also digested with endodextranase from Chaetomium gracile carried out for 16 h at 37° C. with 3 enzyme units per ml of synthesis medium. The digestion products were analyzed by HPAEC-PAD (FIG. 11). The digestion profiles obtained were identical for the four analyzed dextrans, confirming that they all had at least 95% α-1,6 bonds.

The deletions made in the N and C-terminal positions of the DSR-S to construct the DSR-S vardel Δ4N variant thus have no significant influence on the initial activity of DSR-S or on the portion of glucosyl units derived from sucrose incorporated into the synthesis of the HMW dextran, the size or the structure of the polysaccharide.

Rheological Behavior of Dextrans Formed

The rheological behavior of the four dextrans was analyzed using a cone-plane system (AR 1000, TA Instruments) provided with a 4 cm diameter cone at an angle of 3.59°, and covering speeds of 0.01 to 100 s⁻¹. The measurements were carried out at 25° C. Dynamic experiments were carried out in the linear domain between 0 and 10 Pa, with a deformation of 8% for the dextran synthesized by native DSR-S from L. mesenteroides NRRL B-512F (control), 3% for that synthesized by the entire recombinant DSR-S, 5% for that synthesized by a non-purified extract of DSR-S vardel Δ4N and 0.4% for that synthesized by purified DSR-S vardel Δ4N. The complex stiffness modulus is defined by the relationship:

G*(ω)=G′(ω)+iG″(ω).

The energy conservation modulus G′(ω) is larger when the sample is predominantly elastic or highly structured. The loss modulus G″ (ω) represents the energy dissipated during deformation. Predominantly viscous samples have a high G″ (ω).

These rheological analyses produced entirely original results (FIG. 12). As described in the literature, native DSR-S synthesized a dextran with Newtonian behavior [25].

The entire recombinant DSR-S extracts and non-purified DSR-S vardel MN extracts produced viscous solutions with identical behavior (viscosity about 10 times higher than that of dextran produced by native enzyme). When observed with the naked eye, they also had a fairly pronounced stringy behavior. Further, after application of new shear stresses, the behavior of said polymers changed from a solution type to a gel type, which is a a novel property which has been identified for this type of biopolymer. The dextran produced by the native enzyme, in contrast, was not stringy, and its behavior was entirely reversible after application of a second series of stresses (FIG. 12A).

The purified enzyme directly synthesized a polymer having the properties of a highly structured gel (FIG. 12B, modulus G′ much higher than G″), retaining its characteristics through a range of temperatures from 10° C. to 70° C. (results not shown). This behavior is completely different from that of the native enzyme.

Only the preparation of purified DSR-S vardel 44N contained only one active dextransucrase in the extract. Native DSR-S is known to be prone to problems of proteolytic degradation [26] and the purification techniques developed could not resolve that problem [27, 28, 29]. Entire recombinant DSR-S used in the test contained at least two active enzymatic forms, like the DSR-S vardel 44N preparation prior to purification. However, the degraded forms of native DSR-S, entire recombinant DSR-S and DSR-S vardel Δ4N are entirely different. It is currently assumed that cooperation between these different active enzymatic forms present in the medium could be the origin of modifications to the dextran chains, causing these differences in behavior.

EXAMPLE 6 Synthesis of Isomaltose from Sucrose

The capacity of mutant DSR-S vardel Δ4N SEV663YDA to synthesize only isomaltose (IMO with DP 2) from sucrose to the detriment of high molar mass dextrans was studied.

The mutant was purified by affinity chromatography using the procedure described for DSR-S vardel Δ4N given in Example 3.

The activity was assayed at 30° C.

With a specific activity of only 9 U/mg, the SEV663YDA mutations induced severe effects on the activity of DSR-S (loss of 98% of the initial sucrose consumption rate). That specific activity, however, is equivalent to that of recombinant amylosucrase from N. polysaccharea [32], which has been widely studied for its application potential.

The characterizations which were carried out demonstrate the feasibility of producing isomaltose by this mutant DSR-S, while the wild enzyme produces only high molar mass dextrans. Syntheses were carried out at 25° C. in a buffer containing a concentration of 50 mM of sodium acetate at a pH of 5.2 and 0.05 g/l of CaCl₂, 1 U/ml of purified enzyme and using 100 g/l of sucrose as the only substrate, or by acceptor reaction starting with 100 g/l of sucrose and 50 g/l of glucose. Exhaustion of sucrose was monitored by HPAEC-PAD analyses (see Example 4 for analysis conditions) and the reactions were interrupted after complete consumption.

Isomaltose production thus reached a yield of 47% using sucrose as the only substrate (Table 3 and FIG. 13), a yield which was equivalent to that obtained by the acceptor reaction. Adding an exogenous acceptor was thus not necessary. Traces of isomaltotriose, maltose or nigerose (not separated by the system) were also identified (FIG. 13) as well as the presence of other oligosaccharides with a DP of less than 7 and of unknown structure.

TABLE 3 Synthesis of isomaltose by mutant DSR-S vardel Δ4N SEV663YDA from 100 g/l of sucrose alone, or by acceptor reaction with 50 g/l of glucose. Concentration of different products present at the end of the reaction. 100 g/l sucrose + 50 g/l 100 g/l sucrose glucose Glucose 16.73 33.14 Fructose 45.95 42.31 isomaltose 23.99 47.17 Other oligosaccharides 13.33 27.38 % of glucose residues  47.98%   47.17%² transferred to isomaltose ²calculated from glucosyl residues derived from exogenous glucose and sucrose added to medium.

Thus in this Example, the production of isomaltose attained a yield of 47%. Currently, this is the first method involving a single enzyme for synthesizing isomaltose from sucrose; all prior studies being linked to the degradation of starch by a cocktail of α-amylases and glycosidases [11], or to the joint action of dextransucrase and dextranase [30]. Further, sucrose is a cheap and widely available substrate and the fructose released during the syntheses constitute a co-product the value of which can be exploited separately.

EXAMPLE 7 Synthesis of Dextran by DSR-S Vardel Δ3

Different enzymatic forms of DSR-S vardel Δ3 were produced during culture of E. coli TOP10. However, the entire form was vastly in the majority and the zymograms produced (see Example 3) showed that only the entire form was active.

The optimum activity temperature for this variant was 20° C. Thus, activity assays were carried out at this temperature. Production of DSR-S vardel Δ3 in accordance with Example 2 reached about 320 U/l of culture.

Dextran syntheses were carried out at 20° C. in a buffer containing 50 mM of sodium acetate, pH of 5.2, and 0.05 g/l of CaCl₂, 100 g/l of sucrose and 1 U/ml of non-purified DSR-S vardel Δ3 extract. The DSR-S vardel Δ3 extract could be purified by affinity chromatography on nickel resin using the protocol described for DSR-S vardel Δ4N in Example 3. However, since the sonication supernatant contained only a single enzymatic form of dextransucrase and E. coli did not produce another enzyme which could consume the sucrose, purification of the variant did not constitute a prerequisite for rigorous characterization of its properties. By way of comparison, dextran syntheses were carried out under the same conditions as with (non-purified) DSR-S vardel Δ4N. The disappearance of the sucrose was monitored by HPAEC-PAD analyses and the reactions were stopped (5 minutes, 95° C.) after total exhaustion.

The synthesized products were analyzed and quantified by HPAEC-PAD and HPSEC using the conditions described in Example 4. For the HPSEC analyses, the size of the dextrans was estimated using commercially available dextrans with sizes of 2×10⁶, 503×10³, 70,000, 10,000 Da, maltoheptaose and glucose (Sigma).

As can be seen in FIG. 13, at 20° C. the DSR-S vardel Δ3 variant synthesized two populations of polymers; major population of HMW dextran with a size of 2×10⁶ Da, representing about 39% of the glucosyl residues derived from sucrose (Table 4) and a second population of 1,300 to 52,000 Da, centered at the highest peak at around 10,000 Da (about 25% glucosyl residues). This is the first time that a second population of dextran which is clearly visibly on the HPSEC chromatogram has been observed for a DSR-S variant.

Effect of Temperature on the Profile of the Products

Dextran syntheses were also carried out at a temperature of 10° C., still with a buffer containing 50 mM of sodium acetate, pH 5.2, 0.05 g/l of CaCl₂ and 1 U/ml of enzyme (activity assayed at 20° C.). Sucrose exhaustion was monitored by HPAEC-PAD analyses and the reactions were stopped (5 mM, 95° C.) after total consumption thereof.

As can be seen in FIG. 14, at 10° C. the DSR-S vardel Δ3 variant synthesized a population of dextran which was very different from that produced at 20° C. The major polymer (about 44%) formed at that temperature had a molar mass in the range 7,000 and 1.7×10⁵ Da centered at the peak at around 40,000 Da.

TABLE 4 Percentage of glucosyl units incorporated into different products synthesized by DSR-S vardel Δ4N nd DSR-S vardel Δ3 at 10° C. and 20° C. starting from 100 g/l of sucrose DSR-S vardel Δ4N DSR-S vardel Δ3 20° C. 10° C. 20° C. 10° C. HMW dextran > 55.2 37.1 39.2 8.8 2 × 10⁶ Da Dextran nd¹ nd nd 43.9 40,000 Da Dextran 10,000 18.2 14.7 24.8 nd Da Oligosaccharides 16.2 39.1 27.3 36.7 with DP ≦ 8² Leucrose 9.2 7.6 5.3 9.3 Glucose 1.2 1.5 3.4 1.3 ¹nd: not detected ²degree of polymerization calculated from retention time estimated at lower limit of 10000 Da dextran peak.

Effect of Sucrose Concentration

Four increasing concentrations of sucrose were tested (100, 150, 200 and 250 g/l) for the dextran syntheses carried out at 20° C. and 10° C. with DSR-S vardel Δ3 (1 U/ml). The total consumption of sucrose was monitored by HPAEC-PAD analyses and the syntheses were stopped after its total consumption (less than 48 h).

For the two temperatures, the initial increase in the concentration of substrate encouraged the synthesis of low molar mass dextrans. At 20° C., the synthesis of 10,000 Da dextran thus changed from a yield of 25% to 48% on changing from 100 to 250 g/l of initial sucrose. At 10° C. and from 250 g/l, HMW dextran synthesis was completely abolished, and that of dextran with the main population with a molar mass centered around 40,000 Da advantageously reached a yield of 69%.

For all of the dextrans synthesized by DSR-S vardel Δ3, at 10° C. and 20° C., and from 100 to 250 g/l of sucrose, the endodextranase digestion profiles (see Example 5) carried out confirmed that the binding specificity of DSR-S was unchanged (same oligosaccharide profiles detected by HPAEC-PAD as with DSR-S vardel Δ4N, i.e., at least 95% α-1,6 bonds).

EXAMPLE 8 Synthesis of Dextran By DSR-S Vardel Core and DSR-S Core ΔA

The DSR-S vardel Core and DSR-S Core ΔA variants were also slightly degraded during expression by E. coli TOP under the conditions described in Example 2. However, as was the case for the DSR-S vardel Δ3 variant, only the entire form, which was in the vast majority, was active according to the zymogram (results not shown).

The optimum activity temperature for these variants was also 20° C. Production thus reached 38 and 180 U/L of culture for DSR-S vardel Core and DSR-S Core ΔA respectively.

Dextran syntheses were carried out at 20° C. and 10° C. using 100 to 250 g/l of sucrose in a buffer containing 50 mM of sodium acetate, pH 5.2, 0.05 g/l of CaCl₂ and 1 U/ml of enzymatic extract (non-purified). Sucrose consumption was monitored by HPAEC-PAD analyses and the syntheses were stopped (5 mM, 95° C.) after complete exhaustion (less than 48 h). The products formed were analyzed by HPAEC-PAD and HPSEC and their concentration was quantified as described in Example 5.

FIG. 15 shows the profile of the products synthesized at 20° C. by the two variants (HPSEC chromatogram). It can clearly be seen that with these variants, and in contrast to DSR-S vardel Δ4N and DSR-S vardel Δ3, the major population of dextran formed had a molar mass of close to 10,000 Da with the base of the peak between 1,300 and 52,000 (at half height between 5,000 and 22,000). With the DSR-S Core ΔA variant, the synthesis of HMW dextran was completely abolished (Table 5). A reduction in temperature to 10° C. could increase the yields of dextran with ˜10,000 Da without a significant size difference, as was the case with DSR-S vardel Δ3 (Table 5). Dextran synthesis with the DSR-S Core ΔA variant thus reached a yield of 75%. An equivalent yield was obtained with the DSR-S vardel Core variant when the initial concentration of sucrose was 250 g/l (results not shown).

TABLE 5 Percentage of glucosyl units incorporated into different products synthesized by DSR-S vardel Core and DSR-S Core ΔA at 10° C. and 20° C. starting from 100 g/l of sucrose DSR-S vardel DSR-S vardel DSR-S Δ4N Core Core ΔA 20° C. 10° C. 20° C. 10° C. 20° C. 10° C. HMW 55.2 37.1 9.9 2.4 nd Nd dextran > 2 × 10⁶ Da Dextran 18.2 14.7 57.5 62.5 64.4 74.5 10,000 Da Oligosac- 16.2 39.1 19.6 14.8 19.8 10.0 charides with DP ≦ 8² Leucrose 9.2 7.6 6.5 10.2 12.7 12.8 Glucose 1.2 1.5 6.5 10.1 3.1 2.7 ¹ nd: not detected ²degree of polymerization calculated from retention time estimated at lower limit of 10000 Da dextran peak.

HPAEC-PAD analysis of the dextran synthesized from 100 g/l of sucrose at 20° C. by the different variants showed the very high polydispersibility of the product (FIG. 16), containing isomalto-oligosaccharides with a DP of 2 to a DP of about 60 for DSR-S Core ΔA in particular.

For all of the dextrans synthesized by DSR-S vardel Core and DSR-S Core ΔA at 10° C. and 20° C., and using 100 to 250 g/l of sucrose, the endodextranase digestion profiles (see Example 5) carried out confirmed that the binding specificity of DSR-S was unchanged (even the oligosaccharide profiles detected by HPAEC-PAD compared with DSR-S vardel Δ4N, thus at least 95% α-1,6 bonds).

EXAMPLE 9 Acceptor Reaction with Glucose

Acceptor reactions were carried out at 20° C. with a sucrose/glucose ratio of 2 (100 g/l of sucrose, 50 g/l of glucose), 1 U/ml of extract of DSR-S vardel Δ4N, DSR-S vardel Δ3, DSR-S vardel Core and DSR-S Core ΔA in a buffer containing 50 mM of sodium acetate at a pH of 5.2 and 0.05 g/l of CaCl₂. The total consumption of sucrose was monitored by HPAEC-PAD and the reactions were stopped after it had been completely exhausted. All of the variants synthesized isomalto-oligosaccharides (IMO) with a DP of 2 to about 30, to the detriment of the synthesis of polymer with a higher DP.

However, the yields obtained were higher for the variants truncated of A units. Hence, IMO production reached 52% in the case of DSR-S vardel Δ3 and 58% for DSR-S vardel Core and DSR-S Core ΔA, as opposed to 47% in the case of DSR-S vardel Δ4N. The oligosaccharide distribution was also modified (FIG. 17).

For DSR-S vardel Δ3, the proportion of IMO with a DP of 2 to DP of 15 was less than that of products synthesized by DSR-S vardel Δ4N. The situation was reversed for IMOs with a DP of more than 15.

Similarly, the DSR-S vardel Core and DSR-S Core ΔA mutants were shown to perform better for the synthesis of IMO with a high DP than DSR-S vardel Δ4N or native DSR-S (DP essentially 2 to 15): the production of IMO with a DP of 12 to a DP of 27 was two to five times higher with these two variants (according to the ratio of the surface areas obtained by HPAEC-PAD).

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Having described the invention, the following is claimed:
 1. A prebiotic composition comprising dextrans with a controlled size of 1,300 to 5,200 Da centered around 10,000 Da, dextrans with a controlled size of 7,000 to 1.7×10⁵ Da centered around 40,000 Da, dextrans with a high molar mass of 2×10⁶ Da to 109 Da or isomalto-oligosaccharides of 342 to 5,000 Da.
 2. The prebiotic according to claim 1, wherein said dextrans with a high molar mass have non-Newtonian behavior.
 3. A prebiotic composition comprising a dextran with a controlled molar mass obtained by reacting sucrose with a dextransucrase consisting of SEQ ID NOs 6, 7, 8, 9 or 10 or a fragment of said dextransucrase of SEQ ID NO:6 from amino acid at position 1, SEQ ID NO:6 from amino acid at position 125 to amino acid at position 1423, SEQ ID NO:7 from amino acid at position 125 to amino acid at position 1335, SEQ ID NO:8 from amino acid at position 125 to amino acid at position 1136, SEQ ID NO:9 from amino acid at position 125 to amino acid at position 1006, or SEQ ID NO:610 from amino acid at position 125 to amino acid at position
 1423. 4. The prebiotic composition according to claim 3, further comprising reacting said dextransucrase with at least one acceptor.
 5. The prebiotic composition according to claim 4, wherein the at least one acceptor is glucose, isomalto-oligosaccharides, maltose or isomalto-oligosaccharides.
 6. A prebiotic composition comprising an isomaltose-oligosaccharide with a controlled molar mass obtained by reacting sucrose with a dextransucrase consisting of SEQ ID NOs 6, 7, 8, 9 or
 10. 7. The prebiotic composition according to claim 3, further comprising reacting said dextransucrase with at least one acceptor.
 8. The prebiotic composition according to claim 7, wherein the at least one acceptor is glucose, isomalto-oligosaccharides, maltose or isomalto-oligosaccharides. 